Society’s seven day calendar week is the only major rhythm of human activity that is totally oblivious to external nature. This so-called “social week” rests on mathematical regularity alone. We may casually assume that our week is really a division of the moon cycle. If that is our assumption, we forget that the lunar cycle is not a twenty-eight-day cycle, but approximately twenty-nine days, twelve hours, forty-four minutes and three seconds — or 29.5306 days between new moons. A precise quarter of the lunar cycle amounts to the uneven figure of 7.38625 days. So any week using that true length would begin at different times of the day every time the cycle started. There is just no way to neatly divide the lunar cycle into weekly blocks of complete days. 
Then what about the sun? Doesn’t the cycle of seven relate to the center of our solar system? Again, no! The 7-day week is also independent from the annual solar cycle of 365 1/4 days. A “year” of 52 weeks would have just 364 whole days. Nor is the week in harmonic sympathy with the star year of 366 1/4 days. Star days or “sidereal days” are about four minutes shorter than solar days (an observer will see a particular star at the same position four minutes earlier on successive nights). In short, there are no known external rhythms in nature that could explain the near universal existence of the seven day social week.
Yet, the importance of the seven-day week — or heptad, a series of seven — is monumental. Eviatar Zerubavel, in his book The Seven Day Circle (The History and Meaning of the Week), notes that:
“a continuous week, for the establishment of settled life with a high level of social organization [is indispensable] . . . . Only by defining the week as a precise multiple of the day, rather than as a rough approximation of a fraction of the lunar month, could human beings permanently avoid the problem of having to handle loose remainders and, thus, introduce into their lives the sort of temporal regularity that they could never attain with the quasi week.” 
Professor Zerubavel is saying that a regular, predictable week plays a major role in developing our civilization.
THE WEEK IN HISTORY
We take for granted the commonness of a world-wide seven-day week, but that hasn’t always been the case. “Weeks” varying in length from three to nineteen days have existed in past cultures. In parts of Africa three, four (especially along the Congo river), five, six and eight day weeks are found, and always in association with market days. Along the Congo the word for week is the same as the word for market. In North America the Mayas of Yucatan — skilled mathematicians and pyramid builders — had clusters of five-day weeks. In South America the Muyscas had a three-day week, the Persians and Malaysians a five-day week.